1. Field of the Invention
The present invention relates to an image sensing apparatus and a control method therefor, and more particularly, to an image sensing apparatus that senses a subject image using a solid-state image sensor such as a CCD, CMOS sensor, or the like, and a control method therefor.
2. Description of the Related Art
In recent years, image sensing apparatuses such as digital cameras and the like, which use a memory card having a solid-state memory element as a recording medium and which record and playback images sensed with a solid-state image sensor such as a CCD, CMOS sensor, or the like, have been successfully developed and widely diffused. At the same time, improvements in resolution and operating speed with respect to both still image and moving image photography are sought for these image sensing apparatuses. Accordingly, increases in the frequencies of the drive signals for driving the solid-state image sensors that constitute digital cameras and the like, as well as increases in the drive frequencies of analog signal processing circuits, A/D converters, and later-stage digital signal processing circuits continue to come at a rapid pace.
Moreover, recently, in addition to improvements in picture quality such as high picture quality and high definition, there has also come to be heightened demand for easy usage that allows near-perfect photography for a variety of different photographic scenes. Accordingly, in order to track quickly moving objects such as in sports scenes and the like, or to prevent camera shake during indoor photography under low luminance, shutter speeds have become faster. Further, in order to make possible photography in places where the use of flash photography is forbidden, such as art museums and aquariums, even further improvement of sensitivity of the image sensor is sought.
In addition to the solid-state image sensor and its driving circuit, these image sensing apparatuses also have a system control circuit such as a CPU, a memory circuit such as a DRAM, and a display circuit such as an LCD. Each of these circuits operates on different high-frequency clock signals. In image sensing apparatuses in which these multiple clock signals of different frequencies are present, the surface area of the substrate has shrunk as the apparatus has been made more compact to the point where clock interference between the circuits can no longer be ignored. Thus, for example, periodical noise caused by interfering clock frequencies is superimposed on the image signal as diagonal stripes of an even pitch (hereinafter called “beat noise”), and this periodical noise is amplified by heightened sensitivity, thus further degrading the quality of the obtained image.
As a factor causing degradation of the quality of the image in the image sensing apparatus, in addition to the system-generated beat noise described above there is also vertical line column noise caused by the structure of the image sensor. For example, vertical stripes caused by defects in the vertical transfer register in the CCD sensor, as well as smears appearing when extremely bright light strikes the sensor, are known. Moreover, in X-Y address-type sensors such as those typified by the CMOS sensor, typically, from photoelectric converter elements arranged in columns and rows, signals from selected rows are read out through vertical signal lines that are the same for each row but different for each column. Consequently, column noise arises easily due to variation in element characteristics from one column to another.
In the conventional art, in order to extract and cancel out this column noise superimposed on the image signal, the output of optical black pixels composed of a plurality of lines positioned in the vertical direction of an effective pixel area of the solid-state image sensor is integrated and stored in a storage unit as one horizontal period of image data. Then, the stored one horizontal period of stored image data is subtracted from the effective pixel data and the superimposed noise is removed, as is proposed, for example, in Japanese Patent Laid-Open No. 7-67038.
In addition, a technique for canceling out beat noise is proposed, for example, in Japanese Patent Laid-Open No. 2002-152600. In Japanese Patent Laid-Open No. 2002-152600, a multi-channel clamp circuit is provided that has multiple channels of the same number as the number of pixels that corresponds to one period of the beat noise formed by two types of clocks, and which successively integrates the signals of the pixels in the optical black area currently being read in at each of the multiple channels. Then, this per-channel integrated value is subtracted from the input image data and the beat noise is removed.
However, in Japanese Patent Laid-Open No. 7-67038, because multiple horizontal periods of image data are integrated as a single horizontal period signal, only column noise generated in the vertical direction can be extracted. Consequently, this approach cannot cope with system-generated beat noise appearing in the diagonal direction.
Moreover, in Japanese Patent Laid-Open No. 2002-152600, because dedicated detection circuits and integration circuits of the same number as the number of pixels that corresponds to the period of the beat noise are required, the structure of the circuit depends on the length of the period of the beat noise. Consequently as the period of the beat noise lengthens the circuit scale increases and cannot cope.